1. Field of the Invention
This invention relates to stripping tin/lead or tin deposits from copper substrates and more particularly, it comprises new and improved compositions for rapidly stripping tin/lead or tin deposits without substantial attack on the copper substrate, the glass fiber used in printed circuit boards, or on other printed circuit board components, while using materials of substantially lower toxicity and improved biodegradability than solder strippers known in the prior art.
2. Background and Prior Art
Numerous acidic compositions have been patented for use in selectively stripping tin and tin/lead alloys from copper substrates. The great majority of these compositions have been based on the use of fluoride, bifluoride, or fluoborate salts to form soluble tin and lead compounds. The oxidants used in these compositions is typically hydrogen peroxide, or occasionally other oxidants such as nitrobenzene sulfonic acids. U.S. Pat. No. 3,841,905 (1974) to Dixon discloses mixtures of ammonium bifluoride and hydrogen peroxide. U.S. Pat. No. 3,677,949 (1972) to Brindisi et al discloses and claims acidic solutions for selectively stripping tin or tin-lead alloys from copper substrates. These solutions consist of aqueous mixtures of a nitro-substituted aromatic compound having one or more --NO2 groups plus a water solubilizing group attached to the aromatic ring; an inorganic acid giving water soluble tin and lead salts such as fluoboric and hydrofluoric acids; a thiourea to prevent stripped tin from redepositing on the bared copper; and a low molecular weight acidic accelerator such as formic or acetic acid. U.S. Pat. No. 4,004,956 (1977) to Brindisi discloses the additional accelerating effect of an added haloacetic acid such as chloroacetic or bromoacetic acid.
Fluoride salts are toxic and relatively difficult to remove from waste waters. Fluoborate salts are less toxic but are very difficult to remove from waste waters. These materials can interfere with municipal waste treatment processes, and are very undesirable in any recycled water systems. All fluoride and fluoborate salts and acids have the disadvantage of attacking silicaceous materials such as the glass fibers used in printed circuit boards.
Hydrogen peroxide is the most commonly used oxidant, but it suffers from several disadvantages. Hydrogen peroxide is poorly stable on storage or use, and usually requires special additives to decrease its decomposition rate. Concentrated hydrogen peroxide is an extremely hazardous material. Acidic compositions using hydrogen peroxide release heat as they strip tin and tin/lead, increasing the strip rate and the hydrogen peroxide decomposition rate in an uncontrolled manner.
U.S. Pat. No. 3,990,982 (1976) to Dixon discloses that a hydrogen peroxide stabilizer is necessary to reduce the decomposition rate. The stabilizers include organic amines, phosphate complexes, and chelating agents. These stabilizers must be controlled within a narrow range, with too high a concentration inhibiting solder stripping while too low a concentration in ineffective in stabilizing the hydrogen peroxide.